the occlusal surface of an upper first molar: Such a cavity would come to us with a dark spot as its only outward evidence, while examination with the aid of an exploring tine would reveal the fact that it was more or less undermined"backward decay" in other words.

To obtain outline form (our first step) a round bur 8/10 mm. in diameter is placed in the pit while in rapid rotation, and the handpiece swayed to and fro to more readily engage the minute blades. A fair amount of pressure is used until the bur enters the dentin. If, however, it does not do so readily, it should be removed and allowed to cool. After entering the dentin it should be

FIG. 4.

removed from the handpiece and followed with a similar bur 1 mm. in diameter, the orifice being enlarged therewith. This completes the use of the round bur, and is its only use in the excavation of cavities. Any further opening of the cavity found necessary is obtained with inverted cone burs and hand instruments. The cavity should now be extended to include the sharp slopes toward the pit, without making its floor deeper. This broadening is done by undermining the enamel with an inverted cone bur 8/10 or 1 mm. in diameter.

The undermined enamel is then chipped away with straight and binangle chisels and enamel hatchets until the margins are laid in smooth areas, which is all the extension for prevention necessary on this surface, kept habitually clean by the food and saliva.

In this cavity, resistance form and retention form (steps 2 and 3) are ob

tained together. tained together. An inverted-cone bur 1 mm. in diameter, held with its square end against the pulpal wall or floor, is carried entirely around the floor, making a sharp angle at its junction with the surrounding walls. The pulpal wall is at the same time made flat, thereby obtaining the resistance form. Do not make the cavity deeper, merely broader. The cavity should be made as shallow as the removal of all decay and proper seating in the dentin will permit.

No convenience form (step 4) is necessary in so far as access is concerned, though, for a gold filling, one or more convenience points sunk in the surrounding walls only-not in the floor-may be

necessary.

It is well to apply the rubber dam here.

In a cavity of this size there is not likely to be necessity for our fifth step, the removal of remaining carious dentin, as it will more than likely have already been freed of caries by our previous instrumentation. If, however, there is carious dentin remaining, it should be removed with spoon excavators 10-6-12.

The next is the sixth step, or smoothing of cavity walls and beveling of cavosurface angles. This is done by careful paring or planing with a sharp chisel of suitable size, in the direction of the margin of the cavity or around the cavity. In such a cavity as this the inclination of the enamel rods will be toward the cavity, and so the enamel walls may be left parallel with each other without fear that any will be left unsupported, but the cavo-surface angle of the entire margin should be slightly beveled to diminish. danger of fracturing the otherwise sharp margin in placing the filling material.

There now remains but one step, the cavity toilet, which, as mentioned before, consists of wiping or sweeping the cavity clear of all dust and débris.

Class II cavities. In our bicuspid model (Fig. 5) we have an example of class II, or a cavity on the mesial surface of a bicuspid. Remember that the occlusal surface was intact when such a case presented. Remember also that this occlusal portion or "step" represents re

tention form, and should not be confounded with extension for prevention, which latter is exemplified in the proximal portion of the cavity by laying the buccal and lingual margins in the immune areas near the axial angles of the tooth.

Outline form. There are three methods of opening into such a cavity. The first plan is to enter the distal terminus of the groove on the occlusal with a bibeveled drill 8/10 or 1 mm. in width, cutting until the dentin is reached. Enlarge with a larger drill or inverted-cone bur. Follow out the groove toward the mesial, sinking it each time to the dentin and drawing to the surface until the mesial marginal ridge is cut through.

FIG. 5.

Enlarge this slot till the dovetail is formed and has been widened and deepened sufficiently at the mesial to partially uncover the decay in the mesial surface.

The second plan is to place a sharp chisel in the center of the mesial marginal ridge, inclining it first toward the buccal and then toward the lingual, repeatedly tapping it with a mallet until a V-shape is cut in the ridge. Enlarge this by the same procedure until the cavity of decay is reached and uncovered. A bibeveled drill may then be placed therein and made to penetrate the dentin. The cut into the cavity may then be made with the inverted-cone bur. After sufficiently enlarging this the bur should be engaged in the dentin close to the dento-enamel junction, pressing it distally along the line of the occlusal

groove, the slot, and then the dovetail may be cut in the occlusal surface.

The third plan is to place a separator in position, lift the teeth apart sufficiently to admit chisel No. 10, or the smaller width hoe (6 angle), approaching the cavity from the buccal direction. Its position may be gradually shifted to the occlusal as the enamel is chipped away closer to the marginal ridge, until finally the enamel of the ridge itself may be chipped away by holding the instrument in the occlusal direction. An inverted-cone bur may then be used for opening up the occlusal surface, as in the second method. Do not make the mistake of making the occlusal step too narrow. It should occupy about onethird of the occlusal width of the tooth bucco-lingually. This may be done with repeated underminings with the burs and chipping with sharp chisels.

Because of the inclination of the enamel rods toward a groove, the walls in the occlusal step may be made to parallel each other in the enamel as well as the dentin. In this position the outer extremities of the enamel rods will be at the enamel wall of the cavity, while the other extremities will be resting in the dentin.

In cutting the proximal portion to outline form, enamel hatchets 15-8-12 should be used to trim the overhanging enamel from the buccal and lingual walls. Often the straight and binangle chisels may be used for this. Then the gingival wall is cut with a scraping motion until sound dentin is reached. The buccal and lingual walls should be cut well toward the axial angles of the tooth, the extent being determined by the width of the embrasures, and should be sloped outward, in line with the inclination of the enamel rods in this locality. If much cutting is necessary, it may be accomplished with the aid of a small inverted-cone bur introduced close to the dento-enamel junction about the center of the gingival wall, and cuts made buccally and lingually, squaring the buccogingival and the linguo-gingival angles. The bur should then be drawn toward the occlusal in the dentin of both the

buccal and lingual walls, thereby undermining the enamel, which is easily chipped away by introducing the chisels in the buccal and lingual embrasures. The lingual wall should be cut so that its margin may be seen past the approximating tooth when looking across the central incisors at the median line. The buccal wall should be cut to correspond. A very good test is to cut into the embrasures far enough to just permit of the insertion of a chisel, past the approximating tooth, from the buccal and lingual, so that its flat side will be in a plane parallel with the enamel wall. It will be readily seen that this gives a flare to the buccal and lingual enamel walls, made necessary by the enamel cleavage. The strongest seating is given your restoration or filling, however, if the corresponding dentin walls can be made parallel. Do not, nevertheless, cut your cavities too deep in an effort to accentuate this. If your cutting has been correct, the gingival wall in both the enamel and dentin will be on the same horizontal plane. This wall should be laid well past the original contact point, and as near the gingiva as practicable, the best results being obtained if it can be laid beneath it. Beware of leaving any chalky or etched enamel here. The junction of the gingival with the buccal and lingual walls should be sharp and definite in the dentin, but very slightly curved in the enamel.

Resistance and retention form in this cavity require that in the step portion the floor or pulpal wall shall be flat and the surrounding wall parallel, with a slight dovetail effect in outline; in the proximal portion the gingival wall will be horizontally flat, and meet the three upright walls, i.e. axial, buccal, and lingual, practically at right angles. The axial wall should meet the buccal and lingual walls at right angles in the dentin, the buccal and lingual walls being as nearly parallel as possible. Of course these latter in the enamel will necessarily flare, as stated before. The squaring of all line angles (wherever two walls meet) is done with chisels and enamel hatchets.

Convenience form in this cavity is ac

complished with the cutting of wedgeshaped convenience points at the gingivo-axio-buccal and gingivo-axio-lingual point angles, as described before. Let me impress again that these are in no sense "pot-legs," and are accomplished with the smallest inverted-cone bur, No. 33, together with chisels or enamel hatchets. These are needed only for starting gold fillings, though they may not be objectionable for plastic restorations.

Any remaining decay should be removed with the spoon excavators after first applying the rubber dam.

The walls should now be smoothed and the cavo-surface angles beveled in all parts. The latter is accomplished in the proximal portions with gingival margin trimmers having 80 in the formulæ for mesial cavities, and with those having 95 in the formulæ for the distal cavities. A careful examination of these instruments will disclose the logic of this and show how admirably they are designed for this work.

The cavity toilet is performed as before.

Class III (see Fig. 6), you will remember, is proximal cavities on the incisors and cuspids which do not involve the re

moval and restoration of the incisal angle. With men of your intelligence and experience it is not necessary to go into their preparation step by step. A careful examination of the illustration herewith will show that, so far as the dentin is concerned, the cavity is given a triangular or wedge shape, with the gingival wall forming the base, while the apex is formed by the juncture, in a sharp acute angle, of the labial and lingual walls at the incisal portion. The axial wall or floor meets these three surrounding walls at right angles, and convenience points are made in the surrounding walls, not in the floor, at the three corners, so to speak. The outline of the cavity in the enamel is more or less oval, not elliptical. This form in the enamel. combines a knowledge of the direction of the enamel rods with an observance of the esthetic, which you know teaches that curves are more beautiful than straight lines.

One particular feature to which I would call your attention is the laying of the labial margin well on the labial surface. Here we have a combination of convenience form or access with the esthetic, for it is better to expose to view the unmistakable evidence of a good gold filling than the hidden gold filling which gives to the enamel that discolored appearance so easily mistaken for decay and an ill-kept mouth.

It can readily be seen that the sharp angles and convenience points in the dentin establish undercuts sufficient to make it impossible to dislodge a filling. Class IV (see Fig. 7) is in principle a class III cavity in which the decay has progressed so far as to necessitate the removal and restoration of the incisal angle. I have set forth the treatment of such a cavity in the model of a central incisor.

Here you will note we have a rather complicated form, and I have chosen it purposely. So far as the proximal portion is concerned, it is prepared, as far as it goes, after the same principle as the previous cavity-the two gingival convenience points included-with the exception that here you will note a slight

convexity of the floor or axial wall, in an effort to avoid exposure of the pulp. This makes its junction with the labial and lingual walls a little less than a right angle or more acute. However, the labial and lingual walls are erected in planes which are parallel in the perpendicular, but slightly converging as they extend toward the incisal. This gives the secure wedge form which prevents dislodgment toward the incisal, while the trench, so to speak, which is cut in the incisal edge of the tooth, between the labial and lingual enamel plates, together with the convenience point placed at the distal termination and in the floor of this slot or trench, makes it additionally secure

against tipping to the mesial. You will also note that in the incisal step portion more of the lingual plate of enamel is removed than the labial. This is because the stress of mastication is normally from the lingual in the upper anterior teeth. Were this a lower incisor the labial plate would be sacrificed more than the lingual, for the reason that the stress is just the reverse. You will note the "rounded corner" in the enamel at the junction of the proximal with the incisal portions of the cavity. This is to add strength and to prevent fracture with plugger points. Also please note that the incisal step is cut to about the junction of the middle with the distal third of the tooth, which is the point where these respective lobes of the tooth coalesce. This is a weak line, hence we should cut to or past it and not just nearly to it. This incisal step or slot

is cut with the smallest sizes of invertedcone burs which give at once our outline, resistance, and retention forms.

Class v (see Fig. 8) is cavities in the gingival third, not pit cavities, of all the teeth. I am in the habit of referring to these in lecturing to my classes as "filth cavities" a little strong, perhaps, but it drives home the point that most of these could be prevented by the use of a toothbrush and water four times a day.

I have elected to prepare one of these in my models on the buccal surface of an upper first molar. So far as the principles involved in the preparation of this class of cavities are concerned, they are

FIG. 8.

simple. So far as the practical preparation of this class in the mouth is concerned, these cavities are sometimes most difficult, first because of lack of access, and second because of extreme sensitiveness. The hand instruments will be found less painful than the burs. The first strokes are usually the most painful, but if well and vigorously made, most of the decayed material in the central portion can be removed. This should be followed with a sharp inverted-cone bur about 1 mm. in diameter, passed quickly around the periphery, with its square end resting upon the floor or axial wall of the cavity. Cut these cavities to but not over the angles of the teeth, which you will remember are the immune areas. Remember again that the age of the patient must guide you in laying the gingival margin of these cavities. You may find it necessary to use the squareend fissure bur in squaring up the mesial

and distal walls, which are liable to be too much undercut or marred by the inverted-cone bur. These cavities need almost no resistance form, except as it is included in the retention form, for the reason that they are not subjected to any stress. Convenience points for starting gold may be used as required, the maximum number being four, one being placed at each theoretical point angle, but scarcely more than two will ever be found necessary, and often none at all, depending upon the skill of the operator.

I have said so much about extending to immune areas that I want to relate what happens when this practice is ignored. Take, for example, this last cavity (Fig. 8). If the central part is cut out and filled without extension, the growth of micro-organisms will recur on the filling, and being unable to make any impression thereon, the nidus or nest will spread over it mesially or distally, or both, and decay of the contiguous enamel quickly ensues.

CUTTING INSTRUMENTS USED.

I do not feel that my talk would be even nearly complete without some mention of the system of cutting instruments devised by Dr. Black, and the scientific principles on which they are constructed. Time will not permit me to go into the detail of their logical and systematic arrangement, their different uses, etc., but I want to say that, besides having them make a certain number of these instruments, my classes are required to learn, among other things, their arrangement. This is done by class drills wherein some become so proficient as to be able, from the time of a complete disarrangement, to replace each instrument in its proper position in as short a time as four minutes. A complete set consists of 102 instruments; a long set, 51 instruments, and a short set, 25 instruments. The University set consists of 48 instruments, arranged after a definite, logical plan. They are made upon scientific principles, and a descriptive formula is stamped upon each handle. For the straight instruments, as, for example,